Re: cheap way to test "doorknob" capacitors? (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 11 Nov 2004 15:19:10 -0600
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: cheap way to test "doorknob" capacitors? (fwd)

High Voltage list wrote:
> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
> ---------- Forwarded message ----------
> Date: Thu, 11 Nov 2004 07:12:17 -0700
> From: Gomez Addams <gomez@xxxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
> 
> 
> On Nov 10, 2004, at 7:14 PM, High Voltage list wrote:
> 
> 
>>Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>>
>>---------- Forwarded message ----------
>>Date: Wed, 10 Nov 2004 19:31:13 -0600
>>From: Shaun Epp <scepp@xxxxxxx>
>>To: High Voltage list <hvlist@xxxxxxxxxx>
>>Subject: Re: cheap way to test "doorknob" capacitors? (fwd)
>>
>>I read that the capacitance of door knob caps are a function of 
>>voltage.
>>The capacitance is higher at high voltages.
> 
>   Also: I have seen standard barium titanate doorknobs used as tank 
> circuit caps in RF induction furnaces, diathermy machines, and in tuned 
> filter stages of radio transmitters.  How could that be if their 
> capacitance varies with applied voltage?
> 
<SNIP>

Shaun is partially correct, and so is Gomez! HV ceramic capacitors are 
considerably more complex than they seem... and all doorknob caps are 
not the same.  :^)

There are two major "classes" of ceramic dielectric materials used in 
HV ceramic capacitors. Class I ("Temperature Compensated") caps use a 
lower "k" Strontium Titanate based dielectric blend, while class II 
(and class III) caps use a Barium Titanate based higher "k" blend. 
Although class II and III caps have better volumetric efficiency, they 
also have a number of undesirable characteristics as well. Vendors use 
various mixes of materials and binders to help balance capacitance, 
dielectric loss, and stability versus temperature, voltage and 
frequency to better match intended applications. Generally, DC rated 
and low pulse duty caps will use a class II/III dielectric, while more 
demanding RF and pulse applications may require a class I dielectric.

Class II/III caps have significantly higher dielectric dissipation 
than class I. Their capacitance can change dramatically with respect 
to temperature - as much as 70,000 ppm/degree C! Type II/III 
capacitors also decline in capacitance over time (aging), and they are 
also electrostrictive - the capacitor dielectric element changes 
physical size versus applied voltage. These mechanical stresses create 
additional body heating in RF applications. Finally, class II/III 
dielectrics show a significant decrease in capacitance with increasing 
voltage, in some cases decreasing by as much as -80%(!) at full 
voltage. For example, see the charts for R6000 and Z5U Type II 
materials on page 10 of the following document:
http://www.vishay.com/docs/22071/geninfo.pdf

Some companies supply both Class I and Class II/III HV ceramic 
capacitors that physically look identical. For example, Vishay 
Cera-Mite makes a Class III 40 kV cap for DC applications (their DK 
series) and a Class I 40 kV cap in the same physical package for more 
demanding (Tesla Coil, Laser, pulse) applications (their KT series):
http://www.vishay.com/docs/23098/hdhvcaps.pdf

BTW, because of the many problems associated with Class II/III 
dielectrics, class I RF Power Capacitors are specified for the most 
demanding RF transmitting and induction heating applications, and 
higher power units are water cooled. Some examples you might run into 
include:
http://www.vishay.com/docs/22087/tosztosw.pdf
http://www.vishay.com/docs/22132/twxfz140.pdf
http://www.highenergycorp.com/download/ceramic3.pdf

Hope this helped,

-- Bert --
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